1. Field of the Invention
The present invention relates to corrosion protection of pipes, and more particularly to a method for coating pipes using a slot die for the co-reactive extrusion of a two-component thermosetting material.
2. Related Art
The use of tape and coating materials is generally known for protecting pipelines from corrosion. For example, thermoplastic tape or coating materials can be spirally or helically wrapped around the diameter of the pipe and sealed to enclose the outer surface. Large diameter pipes intended for subterranean installation can therefore introduce particular problems of their own due to their size and mass and because they must usually be corrosion-protected at the construction site. Typically, this is done using a laminate tape comprising a bituminous mastic adhesive on a thermoplastic film carrier. The tape can be applied using a spiral wrap (e.g., in the manner of a cigarette roll) or a helical wrap (e.g., in the manner of a tennis racket handle). The pipe is wrapped helically by moving a tape applicator at a predetermined rate linearly along the length of the pipe as it is being continuously rotated. Coating applications, which are similar to this helical wrapping method, employ a wide slit extrusion nozzle for extruding a thermoplastic material such as polyethylene onto the rotating pipe to seal its outer surface.
In contrast to thermoplastic materials, which are softened and usable as coating material upon heating, thermosetting materials are not typically favored for protecting pipe from corrosion. Thermosetting materials such as polyurethane have been applied to pipes in "stringy" bead form, but this provides a relatively slow coverage rate and introduces a number of inconveniences and risks especially on site. The material may, of course, "set up" or harden within the bead die or other extrusion equipment. Since it cannot be unhardened by the application of heat, the thermoset material must be chipped out to be removed, or the die nozzle other equipment must be replaced.
Thermosetting polyurethane has also been spray-coated onto large diameter pipes. However, this practice is inefficient because much, if not most, of the material sprayed does not coat the pipe and otherwise poses a health and environmental hazard.
Accordingly, the present invention provides a convenient and efficient method for slot die extrusion of a thermosetting coating material, avoiding the foregoing disadvantages while achieving an excellent coverage rate around elongated articles such as large diameter pipes.